Electrophoretic imaging employing periodic electromagnetic radiation

ABSTRACT

Method and apparatus for improving image density, contrast and quality and photographic speed in a photoelectrophoretic imaging system utilizing a particulate suspension for forming the image. In the preferred embodiment the method and apparatus stress a thin layer of the electrophoretic suspension of particles in a carrier on an electrode during imaging by applying a discontinuous illumination in conjunction with a constant or discontinuous field.

United States Patent Carol K. Keller Rochester;

Edward Forest, Peniield, both of N.Y. 764,832

Oct. 3, 1968 Nov. 16, 1971 Xerox Corporation Rochester, N.Y.

Inventors Appl. No. Filed Patented Assignee ELECTROPI'IORETIC IMAGING EMPLOYING PERIODIC ELECTROMAGNETIC RADIATION Primary ExaminerGeorge F. Lesmes Assistant ExaminerM. B. Wittenberg Attorneys-James J. Ralabate, David C. Petre and Barry Jay Kesselman ABSTRACT: Method and apparatus for improving image density, contrast and quality and photographic speed in a photoelectrophoretic imaging system utilizing a particulate suspension for forming the image. In the preferred embodiment the method and apparatus stress a thin layer of the electrophoretic suspension of particles in a carrier on an electrode during imaging by applying a discontinuous illumination in conjunction with a constant or discontinuous field.

PAIENTEDunv 1s I9T| 3,620,950

sum 1 OF 4 FIG. 2

83 INVENTOR CAROL K. KELLER BY EDWARD FOREST A T7URNEV V t FIELD W ILLUMINATION TIME FIELD If ILLUMINATION TIME FIG. 9

T FIELD Y ILLUMINATION TIMI? ELECTROPI-IORETIC IMAGING EMPLOYING PERIODIC ELECTROMAGNETIC RADIATION This invention relates in' general to photoelectrophoretic imaging and more specifically to a method and apparatus for improving quality of the images produced.

A new imaging system in which one or more types of photosensitive radiant energy absorbing particles believed to bear a charge when suspended in a nonconductive liquid carrier and placed in an electroded system and exposed to an image radiation configuration has recently been described. See US. Pat. No. 3,384,565 issued May 21, 1968 in the names of V. Tulagin and L. M. Carreira. The particles of this system migrate in image configuration providing a visual image at one or both of the two electrodes between which they are placed. The system employs particles which are photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating radiation by interaction with one of the electrodes. No other photosensitive elements or materials are required, therefore providing a very simple and inexpensive imaging technique. Mixtures of two or more differently colored particles are used to secure various colors of images and imaging mixes having different spectral responses. Particles in these mixes may have either separate or overlapping spectral response curves and may even be used in subtractive color synthesis. In a monochromatic system the particles will migrate if energy of any wavelength within the panchromatic spectrum of the particle response strikes the particle.

It may be that other systems exist or will be discovered or invented that require in their operation suspensions that have some or enough of the properties of the suspensions described herein that this invention can be used thereon to improve such a system and such used is contemplated hereby.

Therefore, it is an object of this invention to provide a method and apparatus for improving photoelectrophoretic imaging systems.

Another object of this invention is to improve the photographic speed of suspensions in inking systems. Still another object of this invention is to improve image quality in certain imaging systems.

A further object of this invention is to improve color saturation in particular color imaging systems.

Another object is to improve images for line screening effects.

These and other objects, features and advantages of the present invention are achieved by presenting discontinuous illumination in conjunction with a constant or discontinuous electrical field across the imaging suspension.

These and other objects and advantages of this invention will become apparent to those skilled in the art after reading the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of an imaging system including means of providing pulsed illumination to the imaging suspension;

FIG. 2 is a plan view, partly broken away, of the apparatus shown in FIG. 1;

FIG. 3 shows a schematic electrical diagram of a circuit for providing pulsed illumination;

FIG. 4 shows schematically apparatus for providing pulsed illumination and discontinuous field; and

FIGS 5-7 schematically illustrate the imaging in the area affected by the field and illumination;

FIGS. 8l2 graphically illustrate illumination and field patterns; and

FIG. 13 shows alternative field means for use in conjunction with a pulsed illumination.

Referring now to FIGS. 1 and 2 there is shown a transparent electrode generally designated 11 which, for illustration, is made up of a layer of optically transparent glass 12 over coated with a thin optically transparent layer 13 of tin oxide commercially available under the name NESA glass from Pittsburgh Plate Glass Company. This electrode is referred to as the injecting electrode or the imaging electrode. To be coated on the surface of the injecting electrode 11 is a thin layer of finely divided photosensitive particles dispersed in an insulating carrier liquid hereinafter referred to as the suspension. The term photosensitive" may be defined as applying to any particle which, once attracted to the injecting electrode will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation.

The term "suspension" may be defined as a system having solid particles dispersed in a solid, liquid or gas. Nevertheless the suspensions described for illustrations are of the general type having a solid dispersed in a liquid carrier.

Above the suspension 14 is a blocking electrode 16 which is connected to one side of a constant potential source 17 through a switch -18. The opposite side of the potential source 17 is connected to the injecting electrode 11 so that when the switch 18 is closed an electric field is applied across the liquid suspension 14 between electrodes 11 and 16. An image projector made up of a light source 20, a shutter 21, a transparency 22 and a lens 23 is provided to discontinually expose the suspension 14 to a light image of the original transparency 22 to be reproduced. The optical transparency of the electrode 11 is shown by way of example and does not affect the scope of the invention herein. Neither does the particular environment shown for imaging, for example, an opague document may be exposed.

The shutter mechanism is illustrative of any mechanism for modifying a constant illumination source to present intermittent exposures to the imaging system. The light passes to the imaging system only through the aperture 24 in the shutter mechanism 21. The aperture is opened by mechanically operating the shutter mechanism 21. This is accomplished by pulsing a solenoid 25 which is pinned to the shutter plate 26 by pin 27. When the solenoid is activated, the plunger moves the shutter plate along guide pins 28 through slots 29 in the shutter plate 26. The slots 29 are positioned so that the plate 26 moves to open the aperture 24 allowing light to pass through the transparency 22 to the imaging system. The operation of the solenoid 25 is rapid and the shutter plate 26 quickly returns to its closed position as illustrated in FIG. 2.

The apparatus described in FIG. 2 is merely illustrative of any mechanism that can intermittently expose the imaging system to an otherwise constant light source.

The electrode 16 is made in the form of a roller having a conductive central core 30 connected to the potential 17. The core is covered with a layer of ablocking electrode material 31 which may be Tedler, a polyvinyl fluoride commercially available from E. I. DuPont de Nemours and Co., Inc. or other material. In this embodiment of the imaging system, the particle suspension is exposed to the image to be reproduced while a potential is applied across the blocking and injecting elec trodes by closing switch 18. The blocking electrode 16 rolls across top surface of the injecting electrode 11 with switch 18 closed during the period of image exposure. A suitable drive or motor M-l causes this movement. The exposure causes the exposed particles originally attracted to the electrode 11 to migrate through the liquid and adhere to the surface of the electrode 16 leaving behind a particle image on the injecting electrode surface which is a duplicate of the original transparency 22.

The electrical action provided by the addition of an alternating potential across the suspension markedly afiects the contrast, background and density of the image formed on the electrode 11. Although cleaning of the electrodes and transferring or fixing of the images formed are not shown, it is contemplated that suitable means known in the art could be used to achieve the desired results.

In place of the constant light source and shutter mechanism of FIG. 1, a pulsed lamp can be used to provide the necessary discontinuous illumination for the imaging system. FIG. 3 shows an illustration of such a source.

FIG. 3 shows one embodiment that may be used to cause a periodic flashing of a lamp for exposing the imaging system. In

the figure there is shown a lamp enclosed in an envelope 32 containing a material capable of transmitting desired wavelengths of radiation at a high intensity to affect the imaging system. Lead-in wires 33 are embedded in the envelope, each lead-in wire bearing an electrode maintained in a spacedapart relationship.

The anode electrode 34 is connected by an electrical conductor 35 to the positive terminal of a capacitor 36 that may be charged through a charging resistance 37 from an energy source such as battery 38 when switch 39 is closed. The other or cathode electrode 40 is connected to the negative terminal of capacitor 36. In combination with the lamp charging circuit is a trigger circuit 41 including, for example, a radio frequency source 42 and an external winding 43.

The trigger circuit provides for an alternative manner to ignite a lamp. For example, capacitor 36 my be charged, as previously described, to a voltage below the breakdown voltage of the particular conditions of the flash lamp 44'. The lamp may then be triggered by means of a trigger circuit 41 transmitting an impulse from radio frequency source 42 to the external winding 43 to cause a partial ionization of the gaseous medium within the envelope 32 making the medium conductive enough to permit the voltage stored in the capacitor 36 to become discharged through the gaseous medium, from the anode 34 to the cathode 40 thereby producing a high intensity radiation in the wavelength regions required.

Many other trigger devices are suitable for use herein. For example, a charged silver strip painted on part of the envelope or charged metallic reflector can also serve as an external trigger electrode causing the lamp to fire. The lamp itself may be filled with xenon, mercury vapor, or any other gases which would present the desired radiometric results for exposing the imaging system. There are presently available xenon lamps having high frequency, low energy flashes that have a lamp life of l flashes to 50 percent of initial output providing 600 pulses per second at 0.008 joules per pulse. This and other lamps may be used with the apparatus schematically described in this application for achieving the results stated herein. Further, apparatus can be added to vary the frequency of pulses.

Pulsed light can also be achieved by a strobotac such as the General Radio Type 153 l-A electronic flash which can be triggered to yield a light flash with a rise time of l x sec. and a duration of 10 X 10" sec to a l/ 10 peak intensity.

FIG. 4 shows alternative apparatus for improving the image quality in a photoelectrophoretic imaging system of the type described in FIG. 1. Here, the blocking electrode 16 transverse the injecting electrode 1 l leaving a residue of the suspension 14 in image configuration. The electrical bias between the two rollers is supplied by a potential source 46 which is activated by a switch 47 during the tranversing by blocking electrode 16 of the injecting electrode 11. The electrode 16 is caused to move across the surface of the electrode ll by any suitable means such as the motor M-2. Following the imaging by the blocking electrode over the injecting electrode, a third electrode 48 shaped as a roller traverses the surface of the imaged suspension 14. The motor M-2 or any other suitable means may be used to initiate and maintain this movement. The electrode 48 is connected to a suitable constant potential source 50 sufficient to maintain the image fonned previously. A second source 52 having a discontinuous potential is simultaneously applied to the roller 48 with the constant potential source 50. The addition of the potential variation causes an improved image with a minimum of background and better range of density and contrast.

A light source 44 similar to that of FIG. 3 is used to illuminate the object 22 for intermittent exposure of the imaging system through the lens 23. The intermittent exposure occurs in timed relationship to the discontinuous field supplied by source 52. This relationship is discussed hereinafter in more detail. By providing an intermittent light source, more intensity of illumination is made practical while limiting the heat generated. This is a significant problem in a machine configuration. Both F IG. 1 and FIG. 4 show a suspension dispenser 54 which schematically represents a means to supply the suspension 14 to the interface between the electrodes forming the image with the suspension 14.

Electrode 11 is referred to as the injecting or imaging electrode and it should be understood to means that it is an electrode which will preferably be capable of exchanging charge with the photosensitive particles of the imaging suspension 14 when the suspension is exposed to light so that a net change in the charge polarity of the particles results. The electrode 16 is referred to as the blocking electrode meaning that it has a tendency not to inject electrons into or receive electrons from the photosensitive particles of the suspension 14. Besides Tedler, which may be used for the blocking electrode, any other suitable material having a resistivity of about 10" ohms per square centimeter or greater may be employed as the blocking electrode surface material.

A wide range of voltages may be applied between the electrodes in the system. For good image resolution, high image density and low background it is preferred that the potential applied be such as to create an electric field of at least about 300 volts per mil across the imaging suspension. The applied potential necessary to attain this field of strength will, of course, vary depending upon the interelectrode gap and upon the thickness and type of blocking material used on the blocking electrode surface. For the very highest image quality the optimum field is at least 2,000 volts per mil. The upper limit of the field strength is limited only by the breakdown potential of the suspension and blocking electrode material. Images produced at fields below about 300 volts per mil, are generally of low and/or low irregular density. The field utilized is calculated by dividing the potential applied between the electrodes by the interelectrode gap measurement. The field is assumed to be applied across this gap. Thus, with two electrodes spaced about 1 mil apart, a potential of about 300 volts applied between the blocking and injecting electrode surfaces will produce a field across the imaging suspension of about 300 volts per mil. Where both the constant and varying potential are being applied simultaneously to the suspension, it is preferable to maintain this low value during the course of imaging.

The particles within the suspension the electrodes nonconductive when not being struck with field is radiation. The negative particles come into contact with or are closely adjacent to the injecting electrode 11 and remain in that position under the influence of the applied electric field until they are subjected to exposure to activating electromagnetic radiation. The particles bound on the surface of the injecting electrode 11 make up the potential imaging particles of the final image to be reproduced thereon. When activating radiation strikes the particles, it is absorbed by 300 particle and makes the particle conductive creating varying of charge carriers which may be considered mobile in nature. These newly created hole-electron pairs within the particles are thought to remain separated before they can combine due to the electrical field surrounding the particle between the two electrodes. The negative charge carriers of these hole-electron pairs move toward the positive electrode ll while the positive charge carriers move toward the electrode 16. The negative charge carriers near the particle-electrode interface at electrode 11 can move across the very short distance between the particle and the surface 13 leaving the particle with the net positive charge after sufficient charge transfer. These net positively charged particles are new repelled away from the positive surface of electrode 11 and attracted toward the negative blocking electrode 16. Accordingly, the particles struck by activating radiation of a wavelength with which they are sensitive, that is to say a wavelength which will cause the formation of hole-electron pairs within the particles, move away from the electron l 1 to the electrode 16 leaving behind only particles which are not exposed to sufficient electromagnetic radiation in their responsive range to undergo this change.

Consequently, if all the particles in the system are sensitive to one wavelength of light or another and the system is exsystem are reversed, electrode 11 will preferably be capable of 5 accepting injected holes from bound particles upon exposure to light and electrode 16 will be a blocking electrode incapable of injecting holes into the particles when they come into contact with the surface of this electrode.

Depending on the particular use to which the system is to be put, the suspension 14 may contain one, two, three or more different particles of various colors and having various ranges of spectral response. In a monochromatic system the particles included in the suspension 14 may be of any color and produce any color and the particular point or range or spectral response is relatively immaterial as long as it shows response in some region of the spectrum which can be matched by a convenient exposure source. In polychromatic systems, the particles may be selected so that particles of different colors respond to difierent wavelengths in the visible spectrum thus allowing for color separation. Regardless of whether the system is employed to produce monochromatic or polychromatic images it is desirable to use particles which are relatively small in size because smaller particles produce better and more stable dispersions in the suspension and are capable of forming images of higher resolutions than would be possible with particles of larger size.

In order for an image to be formed with a pulsed illumination, the images can be produced for given pigments within the suspension only for a pulse width greater than a critical value while for duration time less than this value and image cannot be produced. The duration of the pulse is taken as the required image time. For example, assume a pigment which will produce an image when the duration of a pulse is equal to 6.5 x" 4 seconds or longer. This time requirement is with a pulse having an intensity of approximately 10" photons per cm. 2 per sec and a potential across the electrodes of approximately 1,300 volts. These figures are by way of example only and each pigment under imaging conditions would exhibit dif ferent time requirements and for any given suspension, the requirements of the slowest pigment would be a limiting duration for the pulse for imaging that suspension.

In the illuminated areas, the optical density across the width of the band probably is not constant but increases at the edges. Hence, a simple edge overlap of the bands will produce a periodically varying density and will not necessarily appear perfectly continuous.

If the pulsing of the illumination source is at a repetition rate sufficiently high, overlapping of image bands will be so closely spaced with adjacent image formations that with each flash of light the bands will be so narrow as to appear continuous. There is usually a distinct improvement in the image at this overlap frequency. Pigments tested will form an apparently continuous image with a repetition rate falling between 20 and 30 flashes per second and a roller imaging speed of 1.5 inches per second. The continuous image repetition rate is a function of the roller speed, the width of the field between the roller and the injecting electrode, and the lifetime of the particle.

FIG. 5 depicts the situation set in time when the light flash from the illumination source has terminated. Assume that the lapsed time is less than the image time of the particle and the letter A represents the distance along the electrode 11 over which the field extends. Since the time of the illumination flashed within a simultaneously applied field is less than the image time required, none of the particles which are in the field migrate away from the injecting electrode. This time period is so small that the roller may be thought to remain stationary throughout its duration. The measurement, A, is a function of the geometry of the electrode arrangement and the pigment suspension and is independent of the velocity of the electrode roller.

FIG. 6 represents the same physical events at an instant in time later than the original flash and within the range of image time of the pigments used within the suspension. The roller has moved a distance, V (the velocity of the roller) times t, the amount of time lapsed since the exposure flash. Particles 6-9 have deposited on the roller. Particles 10 and 11 near the field boundary at the rear of the roller and 4 and 5 just entering the field have not had sufiicient time to produce an image since they have not been under the influence of the field for the imaging time.

In FIG. 7, the last of the sequence of F 108., the lapsed time is the lifetime of the pigment particle which is the residual time after illumination that they will image once under the influence of the field. The roller has again moved V4 and particles 3, 4 and 5 have deposited on the roller resulting in a total image bandwidth of C. The distance cleared on the injecting electrode 11 due to the lifetime image is B, where C =A B.

For photoelectrophoretic imaging to occur, these steps take place: 1 migration of the particles toward the injecting electrode 11 due to the influence of the field, (2) the generation of charge carriers within the particles, (3) particle deposition on or near the injecting electrode surface, (4) phenomena associated with the forming of hole electron pairs between the particles and the injecting electrode, (5) particle charge change the injecting electrode, (6) electrophoretic migration toward the blocking electrode. If the field is constant and only the illumination is pulsed, then steps 1 and 2 mentioned above will have already occurred by the time the illumination exposing the suspension flashes.

FIGS. 8, 9 and 10 illustrate the relationships required to form an image if both the field and the illumination are discontinuous. It must be assumed that the pulse width and duration of the field is greater than the imaging time of the particular pigments employed in the suspension. The illumination is indicated by a saw-toothed mark which is similar to that which it presents an oscilloscope. If the pulses illumination and the discontinuous field occur in time in the relationship shown in FIG. 9 no image is produced. However, if the spacing between the pulse illumination and the field is that which is shown in FIG. 10, there will be an image produced provided that the lifetime of the particles is greater than the time between the pulse illumination and the leading edge of the pulsed or discontinued field.

FIG. 11 diagrammatically shows the electrical forces acting on the suspension of a system having an AC oscillation imposed on the field. The ordinate axis represents the potential applied across the suspension between the electrodes 16 and 11 and the oscilloscope reading of the pulsed illumination while the abscissa axis represents time. The lowest dotted line on the graph is the constant voltage required or the Direct Current Ideal necessary to be maintained in order to cause particle migration in image configuration to form an acceptable image. The straight line parallel to the DCI line and designated DCA (Direct Current Actual) represents the actual constant potential that is being put out by a source such as source 17. The distance between the uppermost dotted line and lower most dotted line represents the maximum amplitude of the alternating potential and is called the ACA representing the Alternating Current Amplitude.

FIG. 12 represents graphically the electrical output from a rectified AC circuit superimposed on a constant source. The abscissa axis again represents time while the ordinate represents the potential across the suspension between two electrodes to which this circuit would be connected and the illumination. The DCA and DCI are the same line, it being the solid line parallel to the abscissa. The ACA is that distance shown between the uppermost dotted and the solid line representing the actual and ideal constant potential designated DCA and DCI. The pulsed illumination can occur at every peak or be spaced as shown. There is a benefit with this rectified output to a system that requires a high potential in order to form an acceptable image since the potential variation caused by the varying potential is always in the same direction as the constant potential applied between the electrodes.

FIG. 13 shows a partial view of an alternative apparatus for providing improved image quality in a photoelectrophoretic imaging system of the type generally described in the foregoing figures. Here an electrode such as 80 traverses an imaging electrode 11 after inking of the electrode and the formation of an image by the passage of another electrode over the injecting electrode 11. Of course, the apparatus described could function with either the blocking electrode such as electrode 16 of FIG. 4 or a third electrode such as electrode 48 of the same figure and be within the scope of the invention. A potential source 82 is connected between the electrodes 11 and 80 to cause a field therebetween of sufficiently high potential as described herein to permit electrophoretic imaging while the field generated by the source 82 operates between the electrodes.

An on-off switch 84 is provided in the circuit so that the source is made operable only when the electrode 80 is in use. A motor such as schematically shown and designated M-3 drives the electrode 80 to traverse the electrode 11. A control mechanism 86 permits the intermittent discontinuing field effect between the two electrodes by making and breaking the circuit between the source 82 and the two electrodes. Means 88 for varying the cycle, or the duty time within a fixed cycle, of the voltage applied between the two electrodes can be associated with the control mechanism 86.

Wave forms that are generated by this apparatus are shown in FIGS. 8, 9 and FIG. 10.

When using the methods and apparatus shown above on a photoelectrophoretic imaging system, the photographic speed and color saturation of a polychromatic system or the photographic speed alone of a monochromatic system is improved. In order to achieve these results without a noticeable striation affect, the frequency of the illumination and the varying potential should be determined according to the following formula:

f=VcR Where: f=lhe frequency in Hertz Vc=The relative velocity between the electrodes in millimeters per second and R =resolution desired in line-pairs per millimeter.

The output of the illumination and the amplitude of the frequency used will depend on the response of the pigment particles.

The imaging of the suspension particles occurs only at the image zone between the two electrodes being struck by light and having a field therebetween. By causing a discontinuing field and/or illumination we actually cause particle migrations to occur several times within one contact zone between the two electrodes. As shown in the above embodiments, one electrode traverses another at a particular velocity, V This velocity is generally slow compared to the frequency of the discontinuing sources. Therefore, within any given imaging zone several cycles of frequency variations will occur. It is the spacing between the edges of these cycle bands on the image that is referred to as the resolution of the image formed. Naturally, for a faster relative movement between the electrodes and a fixed given frequency the resolution of the image will decrease. Likewise, for a reduction in the frequency and a constant relative velocity between the electrodes, the image resolution will decrease. By the same formula, of course, the

inverse will cause a higher resolution of the image.

If a visible halftone line screen image is desired, this can be readily achieved by making the frequency f slightly less than V R. This will cause a striation effected image which could be controlled to render a usable halftone line within it.

While this invention has been described with reference to the structures disclosed herein and while certain theories have been expressed to explain the experimentally obtainable results obtained, it is not confined to the details set forth; and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claim is:

l. A method for imaging electrophoretic suspension comprising electrically photosensitive particles including the steps of providing a first electrode adapted to support an image fonned from the suspension,

contacting the suspension with at least another electrode such that the suspension is maintained between said first electrode and said other electrodes,

applying an electric field across the suspension between the first and the other electrodes,

periodically applying activating electromagnetic radiation to the suspension thereby exposing the suspension to an image,

the application of the exposure radiation being applied when the electrodes are in contact with the suspension at contiguous portions thereof and at a time when the suspension will be imageable by the application of the electric field.

2. The method of claim 1 wherein the steps of applying an electric field and periodic application of the exposure radiation occur simultaneously.

3. The method of claim 1 wherein the electric field is also applied periodically.

4. The method of claim 3 wherein the periodic application of exposure radiation and periodic application of the electric field occur simultaneously.

5. The method of claim 3 wherein the periodic application of exposure radiation occurs prior to the periodic application of the electric field.

6. The method of claim 1 including causing relative movement at a predetermined velocity between the first electrode and other electrode.

7. In a method for imaging of photoelectrophoretic particle suspension having a first electrode adapted to support an image formed from the suspension, at least another electrode for contacting the suspension such that it is maintained between said electrodes, including applying an electric field across the suspension between the first and the other electrodes, the improvement including periodically applying activating electromagnetic radiation for exposing the suspension.

8. The method of claim 6 wherein said periodic exposure radiation is applied at a constant frequency.

9. The method of claim 6 wherein application of exposure radiation is of a frequency capable of being varied.

10. The method of claim 6 wherein the frequency of the periodic application of illumination is such that its efiect on the image is greater than 5 line pairs per millimeter. 

2. The method of claim 1 wherein the steps of applying an electric field and periodic application of the exposure radiation occur Simultaneously.
 3. The method of claim 1 wherein the electric field is also applied periodically.
 4. The method of claim 3 wherein the periodic application of exposure radiation and periodic application of the electric field occur simultaneously.
 5. The method of claim 3 wherein the periodic application of exposure radiation occurs prior to the periodic application of the electric field.
 6. The method of claim 1 including causing relative movement at a predetermined velocity between the first electrode and other electrode.
 7. In a method for imaging of photoelectrophoretic particle suspension having a first electrode adapted to support an image formed from the suspension, at least another electrode for contacting the suspension such that it is maintained between said electrodes, including applying an electric field across the suspension between the first and the other electrodes, the improvement including periodically applying activating electromagnetic radiation for exposing the suspension.
 8. The method of claim 6 wherein said periodic exposure radiation is applied at a constant frequency.
 9. The method of claim 6 wherein application of exposure radiation is of a frequency capable of being varied.
 10. The method of claim 6 wherein the frequency of the periodic application of illumination is such that its effect on the image is greater than 5 line pairs per millimeter. 